Because the movie takes place on a movie set and Bruce Lee was the most prominent and best fighting actor/choreographer. You are making it all about race. You could also say why does a random stuntman beat up the most talented fighter. But the answer would be boring: because it’s a movie, not based on reality and the movie wants to picture that guy as a badass.

So Bruce Lee can beat up hundreds of white guys, because asians are good at martial arts, i guess? But there can’t be one white guy in the world who beats up bruce? Racism is complicated indeed.